The Atlantic Ocean Gulf Stream current, which may be affected by future climate change, today takes heat north to Europe, where it cools, sinks and flows southwards at depth, but 10,000 years ago it was weaker and flowed in the opposite direction. Changes in this Atlantic 'meridional overturning circulation' have profound implications for climate.

"[The opposite flow in the Atlantic Ocean] explains the presence of huge ice sheets in Europe and North America during that cold climatic period," said César Negre, an environmental scientist at the Autonomous University of Barcelona in Spain, and co-author of the letter in the British journal Nature. "The ocean currents transport heat, and different configurations of Atlantic circulation have had an impact on its redistribution over the climate system."

The current is the deep-water section of the 'meridional overturning circulation', which is made up of warm surface currents, such as the Gulf stream, that cool as they go. The currents then sink, flowing south along the ocean floor, ventilating the deep ocean and affecting the storage of carbon away from the atmosphere.

The researchers took cores from deep in the Southeast Atlantic, and found that gradient concentrations of two natural radioactive isotopes – Protactinium-231 and Thorium-230 – were the opposite 10,000 years ago from what they are today, showing that the current flowed in reverse.

While surface currents can be driven by a number of things, in deep ocean the driving force is differences in density – and the density is controlled by salinity and temperature.

"It has previously been suggested that increased seawater salinity in the Southern Ocean in combination with ocean surface cooling during the Last Glacial Maximum potentially favoured a reversed deepwater abyssal flow," the researchers wrote in their paper.

In 2005, scientists at the U.K. National Oceanography Centre discovered that the surface current of the Atlantic circulation was slowing down, and climate change models suggest that a slowing Atlantic circulation could create a positive feedback cycle that would further drive climate change.